1. Field of the Invention
The present invention relates generally to a semiconductor device, and more particularly, to a heterojunction bipolar transistor.
2. Description of Related Art
A heterojunction bipolar transistor (hereinafter referred to as a HBT) is a kind of bipolar transistor having an emitter layer made of a material with a wider band gap than a material of a base layer, in which high injection efficiency (emitter injection efficiency) of electrons from the emitter layer to the base layer can be assured even when the base layer has an impurity concentration higher than the emitter layer. Thus, the base layer can have low resistance even with a reduced thickness, and a punch-through phenomenon across the base layer can be prevented to ensure a high emitter-collector breakdown voltage. Basically, the HBT is an excellent device which achieves fast operation and the high breakdown voltage.
The HBT is favorable for use as a device for a power amplifier (hereinafter referred to as a PA) due to high current drive capability. In addition, because of the advantage that the HBT readily operates with a single power source, it has been widely used for a PA in a mobile communication terminal in recent years.
Power-Added Efficiency (hereinafter referred to as a PAE) is known as an indicator for indicating efficiency in a power amplifier. PAE is defined as a ratio of additional power, that is, a difference between an output power Pout and an input power Pin to an applied direct current power Pdc. As the PAE is greater, the power consumption of the power amplifier can be smaller. Thus, the PAE an important indicator in the power amplifier. This is particularly important in a mobile communication terminal in which power consumption of a power amplifier (PA) on the transmitter side makes up a significant portion of the overall power consumption.
FIG. 7 shows an exemplary configuration of a conventional GaAs-based HBT. This semiconductor device includes a subcollector layer 2, made of, for example, n+-GaAs, a collector layer 3 made of n−-GaAs, a base layer 4 made of p+-GaAs, an emitter layer 5, made of, for example, n-InGaP, a first cap layer 6 made of n-GaAs, and a second cap layer 7 made of n+-InGaAs, which are successively stacked on one surface of a substrate 1, made of, for example, semi-insulating single crystal GaAs. An emitter electrode 8 is formed on the second cap layer 7. Mesa structures are formed for forming ohmic contact with the base and the collector such that a base electrode 9 and a collector electrode 10 are in contact with portions of the base layer 4 and the subcollector layer 2, respectively. These electrodes are made of Ti/Pt/Au, for example. The surface of the semiconductor device that is not in contact with any of the electrodes is covered with an insulating film 11, made of, for example, Si3N4.
FIG. 8 shows an exemplary configuration of a. conventional InP-based HBT. This semiconductor device includes a subcollector layer 13, made of, for example, n+-InGaAs, a second collector layer 14 made of n−-InP, a first collector layer 15 made of n−-InGaAsP, a base layer 16 made of p+-InGaAs, an emitter layer 17, made of, for example, n-InP, and a cap layer 18 made of n+-InGaAs, which are successively stacked on one surface of a substrate 12, made of, for example, semi-insulating single crystal InP. An emitter electrode 8 is formed on the cap layer 18. Mesa structures are formed for forming ohmic contact with the base and the collector such that a base electrode 9 and a collector electrode 10 are in contact with portions of the base layer 16 and the subcollector layer 13, respectively. These electrodes are made of Ti/Pt/Au, for example. The surface of the semiconductor device that is not in contact with any of the electrodes is covered with an insulating film 11, made of, for example, Si3N4.
In FIG. 8, InGaAs can be used for the collector layer, but InGaAs has a narrow band gap and thus the base-collector breakdown voltage is reduced. FIG. 8 shows an example of a double heterojunction bipolar transistor (hereinafter referred to as a DHBT) which employs InP in the collector layer for ensuring a higher breakdown voltage. In the DHBT, a conduction-band offset ΔEc occurs between the InGaAs base layer and the InP collector layer to block current from the base layer to the collector layer. Thus, the InGaAsP layer is inserted as the first collector layer 15 between the InP collector layer and the InGaAs base layer to reduce the influence of the potential discontinuity found between the InGaAs base layer and the InP collector layer. For the first collector layer, AlInGaAs or undoped InGaAs may be used.
When the HBT is used to form a power amplifier, one of the requirements for a device to improve the PAE is a reduction in a Knee voltage Vk in Ic−Vce characteristics. Reducing the Knee voltage Vk requires a reduction in offset voltage Voffset which is a threshold voltage of Ic in the Ic−Vce characteristics. The offset voltage Voffset is almost determined by a difference between a forward threshold voltage Vteb between an emitter and a base and a forward threshold voltage Vtbc between the base and a collector (Vteb—Vtbc) . Thus, a conduction-band offset ΔEc produced between an emitter layer and a base layer can desirably be as small as possible.
A frequently used approach for reducing the influence of the conduction-band offset ΔEc is to insert a graded heterojunction, which gradually changes in composition, between the emitter layer and the base layer. However, the graded heterojunction is not necessarily made easily with favorable controllability and reproducibility, and a thick graded layer is needed to eliminate the influence of the conduction-band offset ΔEc and so that holes are not confined completely within the base layer, so that it is desirable to reduce the offset voltage Voffset by lowering the offset voltage ΔEc. It goes without saying that, in the HBT, a valence-band offset ΔEv of the emitter layer and the base layer needs to be large enough to sufficiently block the holes.
In the GaAs-based HBT shown in FIG. 7, an offset voltage ΔEc between InGaP serving as the emitter layer and GaAs serving as the base layer is approximately 0.2 eV. In the InP-based HBT shown in FIG. 8, an offset voltage ΔEc between InP serving as the emitter layer and InGaAs serving as the base layer is approximately 0.2 eV. The values are not excessively high, but a smaller value is desirable.
From the viewpoint of improvement in basic performance of the HBT, a reduction in base resistance is an important challenge. When the base resistance is high, some disadvantages occur such as a reduction in the maximum oscillation frequency fmax and uneven voltage applied between the emitter and the base (emitter crowding) in areas where the current density is high. Thus, the base resistance is desirably reduced as much as possible from the viewpoint of application to a power amplifier.
To reduce the base resistance, the base layer is typically doped at a high concentration to reduce base sheet resistance and base contact resistance. The doping concentration, however, cannot be increased without limitation since the doping concentration has an upper limit and an extremely high doping concentration causes problems such as a reduced current gain and reduced carrier mobility.
As a material of the base layer of the GaAs-based HBT as shown in FIG. 7, a GaAs-based material is typically used. In recent years, C (carbon) is often used as a p-type impurity with less diffusion. The base layer can be doped with C at 1019 cm−3 or higher, but in this case the mobility is as small as approximately 50 cm2/(v·s) or lower.
In the InP-based HBT as shown in FIG. 8, InGaAs is typically used for the base layer. C (carbon) tends to be amphoteric in InGaAs, and the concentration of the p-type impurity cannot be as high as the concentration in the GaAs-based HBT. For this reason, the base sheet resistance is usually higher than in a GaAs-based HBT having the same base layer thickness.
Therefore, to ensure performance equal to or higher than that provided by the currently dominant GaAs-based HBT and InP-based HBT, it is desirable that doping be performed at a concentration that is at least the same level as for the GaAs-based HBT, or that the base layer presents higher hole mobility than in the GaAs-based HBT.